Nuclear reactor



E. F RISCH NUCLEAR REACTOR July 13, 1965 7 Sheets-Sheet 1 original Filed sept'. 19, 1956 lll /l/ lll/l Il l/l /l/ /l//l/ l l/ lll/ll /l/l/l//l/ E. FRISCH NUCLEAR REACTOR July 13, 1965 7 Sheets-Sheet 2 Original Filed Sept. 19, 1956 lNvENToR Erling Frisch BY ATTORNEY July 13, 1965 Original Filed Sept. 19, 1956 E. x--RlsmiI NUCLEAR 'REACTOR Fig. 3.

7 Sheets-Sheet .'5

E. F RISCH NUCLEAR REACTOR July 13, 1965 '7 Sheets-Sheet 4 Original Filed Sept. 19, 1956 E. FRISCH NUCLEAR REACTOR July 13, 1965 7 Sheets-Sheet 5 Original Filed Sept. 19, 1956 u n 2W w o ...w .5 M B 3 9 3 Cow 33 E. FRISCH- NUCLEAR REACTOR July 13, 1965 '7 Sheets-Sheet 6 Original Filed Sept. 19, 1956 Fig.9.

Eqi

"HMM s @g1 J l l Ili'l i s i P H n, V !w\\\\\\\\\\\\\\\\\\ s||.-.--212` m m 4 f A l 42B l0 2 1 n., M 4 O 4 2 2 f f f 4% 6 J 3 O ...Y HV. 4, 4 -Y \\Mn-a HH z t .v5 i w E. FRISCH NUCLEAR REACTOR July 13, 1965 7 Sheets-Sheet 7 Original Filed Sept. 19, 1956 United States Patent O M 3,194,740 NUCLEAR unseren lirling Frisch, Pittsburgh, Pa., assignor to Westinghouse This application is a continuation of copending application Serial No. 610,821 tiled Sept. 19, 1956, now abandoned, in the name of E. Frisch, entitled Nuclear Reactor and assigned to the instant assignee.

The present invention relates to nuclear reactors, and more particularly to a pressurized Water reactor adapted to be operated either for testing or for power purposes.

in a nuclear reactor of the character described a neutron-issionable isotope, such as Um, U235, or Pu239, or mixtures thereof is subjected to fission by absorption of thermal neutrons, With the result that a self-sustaining chain reaction within the isotope is established by an excess of neutrons evolved by the fission. In general the reactor `comprises a number of fuel elements lof fissionable material, for example natural or enriched uranium encased within a suitable protective covering. The fuel elements are disposed in a neutron slowing material which slows the fast neutrons evolved in each atomic fission to thermal energy levels thereof, at which the neutrons are most ecient to induce fission within the uranium or other atomic fuel. T he slowing material is termed a neutron moderator and preferably is formed of a substance having the characteristics of relatively small neutron-ic capture cross section and relatively large scattering cross section. The heat evolved by the chain recation is removed generally by passage of a suitable coolant through the reactor core in heat exchange relationship with the fuel elements disposed therein. Specific details ofthe operational theory of such reactors are set forth in Enrico Fermi and Leo Sziland Patent No. 2,708,656, dated May 17, 1955.

Although the nuclear reactor forming a subject of this invention will be described primarily in arrangements thereof adapted for testing purposes, it will be apparent as this description proceeds that the reactor disclosed herein can be readily operated to produce electrical or other forms of usable power.

Heretofore, nuclear reactors frequently have comprised very elaborate and expensive core and control rod assemblies that are difficult toproduce and to maintain. Be'- cause of the radioactive nature of the atomic fuels used therein and because of the complicated mechanical structures utilized to suspend the fuel elements within the core assembly, the fuel elements contained in prior reactors have been difficult and hazardous to Withdraw for purposes of replacing the atomic fuels therein. Moreover, the arrangements utilized in prior reactors have limited the percentage burnup Iof the atomic fuels attainable in those reactors. The fuel elements themselves have been assembled usually into a bundle thereof by painstaking welding processes which further increase the expense, complexity,and skill required in fabrication in prior reactor cores. Furthermore, the extensive amount of operator `contact with radioactive materials encountered in the fabrication of prior reactor cores greatly increased radioactive hazards to personnel engaged in assembling the reactors.

ldfd@ Patented July i3, 1965 CCv rhe prior reactors of the character described have suffered from the further disadvantage that no eiiicient means have been provided for compensating for neutronic uxile perturbations occasioned within the reactor upon the Withdraw-al of the control rods relative to the reactor core. Thus, the withdrawal of the control rods associated with prior reactors has produced an uneven neutronic fluxile density within the reactor core, which operates to diminish the percentage burnup of atomic fuel otherwise attainable and tends to produce uneven heating or hot spots Within the reactor core. As a result, the life of prior reactor cores has been unnecessarily shortened and the control thereof essential to produce an even heating of the fuel elements therewithin has been diiiicult and hazardous.

The fuel arrangements frequently used in prior reactors necessitated using enormous amounts of water or other `coolants to maintain the reactor core at selected operating temperatures. In the case of forced circulation cooling, considerable amounts of power must be expended in cir-- culating the coolant, and Where the coolant is used also as a moderator, as in prior pressurized reactors, the expenditure of power is increased even further. The customary use of the same fluid material for both coolant and moderator material in prior reactors not only has increased the total volume of liquid which has to be pumped through the nuclear reactor, but has hitherto resulted in the selection of a single material embodying necessary but inadequate and undesirable compromise among the characteristics of low neutron capture cross section, heat conductivity, non-corrosiveness, and the like. Moreover, the reactor fuel arrangements utilized heretofore have rendered difficult any variation in peak power output attainable from the nuclear reactor as by varying the coolant to fuel ratio of the reactor core.

ln order properly to select structural and other materials for use in nuclear reactors it is necessary to test such materials under the operating conditions to which the materials will be subjected, inasmuch as comparatively little is known concerning the neutron absorption and reflection properties, radiation stability, high temperature corrosion and erosion resistance in various coolants and moderators, and related properties of many materials which may be proposed for use in fabricating and operating nuclear reactors. Such materials have heretofore been selected largely on a trial-and-error basis, and no convenient arrangement has been provided for testing such materials at positions of high iiuxile density and under conditions of high temperature and pres-sure within the reactor core without necessitating opening the reactor vessel or core in order to insert or to remove the test specimens.

Accordingly, an object of the invention is to furnish a more ciicient and relatively inexpensive nuclear reactor system, and particularly the core structure thereof.

Another object of the invention is to facilitate cooling the core of a nuclear reactor.

A further object of the invention is to improve vthe fuel element arrangement within a reactor core.

Still another object of the invention is to increase the percentage burnup of atomic fuel utilized in a nuclearv reactor.

Yet another object of the invention is to provide improved testing means associated with the core of an atomic reactor. Y

Another object of the invention is to furnish an im- V proved fuel element for ya nuclear reactor.

A further object of the invention is to facilitate variation in peak power output and neutronic fluxile Idensity of a reactor.

Still another object of the invention is to reduce shutdown time required to change or to rearrange the fuel elements of an atomic reactor.

Still another yobject of the invention is to improve means for handling the fuel elements of a nuclear reactor.

Yet another object of the invention is to provide means for efficiently compensating undesirable neutronic iluxile perturbations within the reactor core occasioned by withdrawal of the contr-ol rods therefrom.

Another object of the invention is to facilitate shielding an atomic reactor.

The aforementioned and other objects, Ifeatures and advantages of the invention will be made more apparent during the ensuing description of illustrative forms thereof, 'which description is to be taken in conjunction with the accompanying drawings wherein:

FIGURE 1 is a vertical sectional View of `one lforrn of `an `atomic reactor constructed according to the principles of this invention.

FIG. 2 is an enlarged sec-tional view yof the reactor core taken along reference lines II--II of FIG. 1.

FIG. 3 is a sectional View taken along reference lines III-III of IFIG. 2.

lFIG. 4 is an enlarged, foreshortened, sectional view of one of the fuel elements utilized within the reactor core, which View is taken along reference lines IV-IV of lFIG. 5.

FIG. 5 is an end view of FIG. 4.

FIG. 6 is a sectional view taken along reference lines VI-VI of IFIG. 4.

FIG. 7 is 4an enlarged, tripartite, longitudinal sectional view of one Iform of control rod assembly utilized Within the atomic reactor of the present invention.

FIG. 8 is a diagrammatic elevation view partly in .section of the control rod assembly.

FIG. 9 is an enlarged, bipartite, longitudinal sectional -view of one `form of high pressure testing thimble utilized within the reactor core.

FIG. 10 is 4a longitudinal sectional view of a fuel element handling tool shown in engagement `with one yof the lfuel elements.

FIG. 11 is a sectional view taken along lines XI-XI of VFIG, 10, and

FIG. 12 is a bottom view of the handling tool taken along the lines XII-XII of FIG. 10.

In accordance Iwith the invention, a nuclear reactor is provided, wherein the core thereof is submerged under a head of circulating coolant and means are associated with the core, in one application of the invention, for `separating the cooling and moderating portions of the circulating iiuid or ilu-ids. The fuel elements supported within the core of the reactor are furnished in a novel form which, in conjunction with means provided by the invention for supporting the elements, permits facile removal or insertion of the fuel elements, thereby facilitating assembly of the core and minimizing shutdown time required to replace, interchange, -or recycle the fuel elements. In -order to minimize perturbations in the neutronic flux within the reactor core, means are provided whereby a fuel element is inserted progressively into the reactor core lwhen the control rod is with-drawn, as in starting up the reactor, or when otherwise manipulating the control rod to control the reactor during operation thereof.

In other applications o`f the invention the reactor core is provided with a number of testing facilities whereby samples of various structural, cooling, and moderating m-aterials can lbe tested under differing conditions of temperature, pressure, and fluXile density. Additionally, means are provided Vfor facilitating handling of the fuel elements and for improving the circulation of coolant therethrough. The structure of the fuel elements and their arrangement within the core is such that the peak power output or iluxile density of the reactor core can be changed readily by varying the number or the arrange- 5 ment 4of the fuel elements within the reactor core or by easily adding or .subtract-ing atomic fuel from the individual -fuel elements.

Referring now more particularly to FIGS. 1 to 3 of the drawings the exemplary form of the invention shown 10 therein includes a reactor vessel 20 supported within a radioactive shielding structure indicated generally at 22 and comprising, for exam-ple, concrete and a densifying material lsuch as magnetite. The reactor vessel, shielding, and associated parts are desirably confined within 15 a vapor-tight enclosure 23, or its equivalent, adapted for preventing the escape of radioactive gases `or dust into the atmosphere. Secured within the reactor vessel is an inner cylindrical member or supporting baille 24 terminating at the `upper end thereof in la frustoconical portion 20 `or flange 26 whereby the cylindrical member 24 is secured to the inner Walls of the reactor vessel 20. Adjacent the other 'end of the cylindrical member 24 is secued a core supporting plate 23 upon which -a reactor core 30 rests. A number of thermal shields 31 are mounted between the cylindrical member 24 and the vessel 20 in positions adjacent the reactor core 3l). The thermal shields 31 are supported by brackets 32 in spaced relation to one another and to the cylindrical member 24 and vessel 20 for purposes hereinafter made apparent. The core supporting plate 28 is furnished with a relatively large central aperture 3-3 through which there protrudes from the reactor core 30 a plurality of control rod lower shroud tubes 34 and a number of rtubular casings 3S, 36 and 37 provided for the insertion of various 35 testing thimbles into the reactor and presently to ybe described in greater detail. The tubular casings 35, 36 and 37 .pass through the biological shielding members 38 and 40, which are secured relative to the reactor vessel 20 by means .of a supporting plate 41. A vertical trans- 40 fer tube 42, secured by the shielding member 4) and by the core supporting plate 23, extends downwardly through the bottom portion of the reactor vessel 20 together with the tubular casings 35, 36 and 3-7. Casings 35, 36 and 37, are secured and sealed in a well-known manner relative to :reactor vessel 2t) at points yof their junction `with .suitable apertures .in the hermetically sealed bottom closure 44 of the reactor vessel 20 by means of the sealed joints indicated generally Iby reference character 45. A number of testing thimbles, presently to be described, are .inserted through the casings 35, 36 and 37 and into the reactor core.

The upper end of the reactor vessel 2) is hermetically sealed by a removable closure 46 upon which are mounted a number of conventional control rod drive mechanisms 48 and a lifting lug 50. The drive mechanisms 4? are arranged to cooperate with a plurality of control rod drive-shafts 52 which are supported within individual upper guide sleeves 54 secured to the closure 46 and within individual lower guide sleeves 56 which are secured to a control rod drive shaft enclosure 53 suspended from the underside of the removable vessel closure 4e. Control rod assemblies 60 are secured to the respective drive shafts 52 by means of scram magnets 62 (FIG. 7), presently to be described in greater detail. Therefore, the control rod assemblies 6@ can be scrammed or detached from the drive shafts 52 in order to terminate the chain reaction in the core 30 in case of an emergency or accident, or to permit removal of the closure 46 together with the drive mechanisms 48, guide sleeves 54 and 56 and the associated drive shafts 52. Thus, convenient access can be had to the interior of the reactor vessel 20 and to the core 30 for adding, replacing or rearrangeing fuel elements 64 (FIGS. 2 and 3) in a mann-er presently to be described.

As better shown in FIG. 3, the fuel elements 64 are spacedly supported within reactor core sleeves 66, which sleeves are secured within the reactor core between end plates 68 and 7?. The end plates 68 and 7i) are bolted or hermetically sealed to casing 72 of the core 3i), as by annular welds 74 and 76. The upper end plate 68 is provided with a plurality of apertures 78 of a size suitable for theadmission of the fuel elements 64 and of the control rod assemblies 6l). The bottom end plate 7i) is furnished with a like plurality of apertures S0 disposed in alignment with the apertures 7S of the upper end plate but having a smaller diameter, except as hereinafter noted in connection with apertures 78a, than the first-mentioned apertures, in order to provide support for the fuel elements 64 which are inserted vertically into the sleeves 66 of the reactor core 30. Both ends of the fuel elements 64 are shaped to engage a chamfer 82 provided adjacent the inner end of each of the bottom plate apertures 8i). The fuel elements 64 are furnished with means presently to be described for permitting the flow of water through the reactor core sleeves 66 in order to aiord cooling of the fuel elements 64.

The reactor core sleeves 66 are maintained in alignment with one another and with the reactor core 30 as a result of their being inserted into counterbored portions 84 and 36 of the apertures 78 and 80, respectively. The counterbored portions $4 and S6 may be drilled further through the end plates 63 and 70, respectively, and the length of the sleeves 66 may be increased accordingly, to facilitate hermetically sealing, as by welding, the sleeves 66 at each end thereof to the end plate 63 or 70, Where such hermetic sealing is required as will subsequently be pointed out. Thus, each of the apertures 76 in the upper end plate 68 is coupled to a matching aperture 8i) in the bottom plate 70 by means of the reactor core sleeves 66.

A plurality of control rod assemblies 6i?, nine being shown in this example of the invention (FIG. 2), are insel-table through selected ones of the apertures 73 and Sil and associated reactor core sleeves 66 at suitable positions among the fuel element lattice of the reactor core Sil. The control rod assemblies 60 are insertable through the reactor core 39 by means of upper shroud tubes t? and the lower shroud tubes 34 threaded, respectively, into the tapped portions 92 and 9d of certain of the apertures 7S and 8i).

For testing various materials under conditions of high pressure and temperature and at areas of maximum neutronic fluxile densities within the reactor core, the centrally disposed tubular member 35 extends upwardly through the reactor core 39 via apertures 96 and 98, provided, respectively, in the upper and lower end plates 62 and 76. The upward end of the tubular member 35 is closed by a cap 11MB which can be hermetically sealed to the tubular member 35 in those cases where it is desired to prevent leakage of uid from the reactor core into the tubular member 35. A high pressure testing thimble 102, terminating in rod-like proiection 164 and having testing specimens indicated generally by the cylindrical body 106 hermetically sealed therein, is inserted upwardly into the reactor core 39 from underneath the reactor vessel 20. A space 168 (FIG. 1), is provided within the concrete shield 22 and below the reactor vessel 20 for the purpose of receiving suitable machinery and affording ample workspace for handling and inserting the high pressure testing thimble 162 and similar testing units presently to be described. The rod portion 164.- secured to the upper end of the testing thimble 162 is arranged to be inserted into an annular projection 110 on the cover 100, for the purpose of centering thetesting thimble 162 relative to the tubular casing 35 and the reactor core 36.

in a similar manner the tubular casings 36, six being utilized in this example of the invention, protrude upwardly through apertures 112 and 114 provided, respectively, in the upper and lower end plates 68 and 7@ and are sealed at the upward ends thereof by caps 116. lnsertable into each of the tubular casings 36 from a position within the space 193, and in a manner similar to that described in connection with the high pressure testing thimble 162, is a high pressure testing thimble 118 similar in structure to the aforesaid thimble 162. The tubular casings 36 and associated testing thimbles 118 are desirably placed in areas of high neutronic fluxile density within the reactor core Si).

Adjacent the outer periphery of the reactor core 30 an additional testing facility is provided within the reactor core Sil comprising the rabbit tubes or casings 37, six being utilized in this example and arranged for the quick insertion or withdrawal into the reactor core 3) of a number of sealed tubular sections 120, which can be quickly inserted into the tubular casings 37 from the space 1&3 beneath the reactor by a suitable valve and pressurized fluid arrangement. The rabbit facilities or casings 37 are provided of such diameter to form a loose t with the apertures 78a and Sila provided, respectively, in the upper and lower end plates 63 and ill of the reactor core. The tubular casings 37 are thus aligned relative to the associated reactor core sleeves 66 by means of the apertures '73a and Silo. The upward end 122 (FIG. 2) of each tubular casing 37 is provided with a vent 128 to relieve pressures developed forwardly of the casings 12@ during insertion thereof into the reactor core, and to permit the passage of coolant through the casing and around the loosely fitting tubular sections 126.

For purposes of cooling the fuel elements 64 and for removing the thermal energies developed therein for conversion to other forms of energy, in the case of power reactors, suitable coolant is pumped into the reactor vessel 26 via an inlet conduit 134. The coolant flows downwardly of the vessel 2l); and in one application of the invention all of the coolant flow is directed entirely through the reactor core sleeves 66 being used, the unused sleeves 66 being closed at the upper ends thereof by means of solid plug members 136 inserted into the associated apertures 7S in the upper end plate 66 of the core 3i). The plug members 136 are provided with spheroidal knobs 137 for purposes presently to be enumerated. Since the coolant ow is downwardly of the core 3G, no means is required to secure the fuel elements 64 and the plugs 136 other than gravity and the support provided by the associated end plates 63 and 76. Thus, in adding, removing or rearranging the fuel elements 64, the same merely need be lifted out of the sleeves 66 and replaced as desired by means presently to be described, thereby greatly facilitating refueling the reactor and minimizing the time lost in this operation. As indicated heretofore, the coolant enters the apertures 73 of the upper plate 68, which are not closed by plugs 136 and flows downwardly through the sleeves 66 and around and through the fuel elements 64 to effect cooling thereof and out through the apertures 89 of the bottom plate 7?. The coolant Ithen passes upwardly through annular passages 133 formed by the cylindrical member 24', the walls of the reactor vessel 29, and thermal shields 31, leaves the reactor vessel 29 through an outlet duct 139. The flow of coolant through reactor vessel 2li is indicated by ilow arrows Mtl and 141.

As will be apparent from the foregoing, the space enclosed within the reactor core 36 and between the reactor core sleeves 66 can be hermetically sealed from the interior of the reactor core sleeves 66 and consequently from the coolant passing therethrough. Thus, in one application of the invention, an eliicient moderator Huid, ie. one having low neutronic absorption and high neutronscattering power, can be maintained within the reactor core 36 but can be prevented from contacting an eicient coolant fluid, i.e. one having satisfactory heat transfer characteristics and passing through the sleeves 66. Since the moderator fluid will not then contact directly the fuel elements 64, the comparatively small amount of heat produced therein will reduce greatly the required volume of iiow thereof through the reactor core 30. Thus, only a small proportion of the total uid within the reactorcore 30 need be changed constantly in order to cool the fuel elements 64, and consequently the flow of coolant will take place only through those reactor core sleeves 66 which are occupied by the fuel elements 64 and the control rod assemblies 60, as will be described presently in greater detail.

For admitting and removing moderator fluid relative to the core 30, inlet and outlet conduits 142 and 144 are provided, which conduits extend outwardly through the cylindrical member 24, the reactor vessel 20, and the concrete shielding 22. With this arrangement whereby the moderator is maintained in a chamber which is effectively separated from the coolant stream, it will be apparent that a moderator can be utilized which is different in composition from that of the coolant. For example, light water or H2O can be utilized for coolant, since a comparatively large volume will be required, whereas heavy water or D20 can be utilized for the moderator material, which is required in much smaller volume than that of the coolant stream. Accordingly, heavy water, which is a considerably more effective moderator, but also a much more expensive material, than light water, can be utilized in the most eflicient and inexpensive manner Within the nuclear reactor. The use of heavy water permits the use of natural uranium as the atomic fuel, whereas less efficient moderators require the use of more costly, enriched uranium. Thus, a material having effective moderating characteristics, but poor heat trans- -fer characteristics, for example light and heavy water, carbon dioxide, or an organic material, can be utilized within the reactor core 3i) but outside the reactor core sleeves 66; while another material, for example a liquid metal, having excellent heat transfer characteristics but a relatively poor moderating characteristic, can be utilized for cooling and energy transfer purposes and can be conned within the reactor core sleeves 66.

Alternatively, the conduits 142 and 144 can be omitted, and a number of small dow holes (not shown) can be provided in each end plate 68 or 7i) and spaced from the apertures 78 and 80, respectively. It is contemplated that the flow holes will be of such size as to divert a small proportion of the coolant stream entering from the conduit 134 into those portions of the reactor core 30 between the sleeves 66. This portion of the coolant stream then serves as the moderating fluid in those applications of the nuclear reactor wherein it is desired to utilize a single moderator-coolant iiuid. The iiow of fluid through the aforesaid flow holes need be only sufficient to prevent stagnation (and overheating if the reactor be used for power purposes) of the fluid wit-hin the moderator section of the reactor core 30.

Referring now t-o FIGS. 4 to 6 of the drawings, there is shown therein one form of fuel element assembly .adapted for use with the nuclear reactor of the character described and constructed in accordance with the teachings of the present invention. One such arrangement of the fuel element 64 comprises, in this example, three coaxi-ally nested tubes 209, 202 and 204 and upper and lower brackets 206 and 26?. Each of the brackets 2Go and 268 includes 4a number of outwardly extending spokes 2li@ provided on the underside thereof with tabs 212 adapted to be inserted between the nested tubes 200, 202 and 264 to maintain the tubes in coaxial alignment with one another in order to afford cool-ant passages 2te therebetween. The upper and lower brackets 295 and 20S are maintained in clamping relationship with the coaxial tubes 2%, 292 and 204 by clamping members 2id insented respectively through central apertures 231.5 of the brackets 266 and 208, and threadedly engaging a centrally disposed supporting tube 21S. The supporting tube ZES is aligned coaxially by the clamping member 226 with the linnermost nested tube 204 in order to provide an additional coolant passage 2l9 through the fuel element 64. The clamping members are each provided with `a shoulder 220 engaging the brackets 266 and 208, respectively, and with an outwardly extending spheroidal knob 222i to facilitate handling the fuel elements 64, for example, in a manner presently to be described. At least the Kouter end portions of the spokes 2l() are inclined or are chamfered `as indicated by the reference character 222 in order Ito properly seat against the 'chamfered portions 82 (FIG. 3) of the apertures 80 in the bottom end plate 70, as noted heretofore. Each of the spokes 210 protrude outwardly of the outermost nested tube 294i of the fuel element assembly, as shown at 224, in order to properly space the fuel element assembly 64 relative to the associated reactor core sleeve 65 FIG. 3.

Each of the nested tubes 206, 262 and 204 includes an inert section 226, an inert section 223 and a core section 236i. Embedded within each of the core sections 2st) is a layer of atomic fuel 232 including, in this example of the invention, an alloy of 13% of enriched uranium and -a balance of aluminum. The upper inert section 226 dcsirably is made substantially equal in length to that of the lower inert `section 228 in order that the core section 230 will he suspended centrally of the reactor core Sii. The atomic fuel 232 preferably is metallurgically bonded to the `material of the concentric sleeves 20d, 292 or 204 in order to ensure adequate heat transfer from the atomic fuel 232 to the coolant passing through the passages 214 and 219. Such metallurgical bonding is facilitated by fabricating the inert material of the -tubes 26), 262, 224 from material identical to that comprising the major portion of the uranium alloy, with aluminum being utilized in this example.

It has been indicated heretofore that the peak power output `of the reactor or the maximum neutronic fluxile density therein can be changed by removing some of the fuel elements 64 or by inserting additional fuel elements into those holes 78 which `are indicated las being vacant in FIG. 2. On the other hand, change in the power output or tluxile density or the distribution thereof within rthe core 3@ can be effected 'by adding or subtracting one or more of the nested tubes Zili), 262 and 204 in some or all of the fuel elements 64. Since the fuel elements 64 are symmetrical along the longitudinal axis thereof, increased burnup of 'the fuel elements 64 can be attained by withdrawing and reinserting the fuel elements 64 in an inverted position Within `the reactor core 30, as well las by the customary rearrangement of the fuel elements ments in accordance with radial luxile variations within the reactor core. This follows from the fact that it has hitherto been determined that neutrcnic fluxile density within a given reactor core is uneven along the length of the fuel elements suspended therein resulting from the usual partial insertion of the control rods into the reactor core. Inasmuch as the fuel elements 64 can be removed one by one from the reactor core 30 without the necessity of disassembling the core 36, it will be apparent that one or more of the fuel ele-ments 64 can be withdrawn from the core 30 for inversion or rearrangement therein without disturbing the balance of the fuel elements `64.

Referring now to FIGS. 7 and 8 of the drawings one form of control rod assembly `adapted for use with the reactor core Si) and constructed in accordance with the teachings of this invention is shown therein. In this example of the invention the control rod assembly 6) com' prises a scram magnet housing 399, secured at the upper end thereof to one of the control rod drive shafts 52, and arranged for insertion through the lassociated lower guide sleeve 56 (FlG. l). Mounted within the scram magnet housing 3% is a scram magnet coil 392 wound upon a suitable core 3&4 comprising a uxible gap formed by ya non-magnetic ring member 3&5, and having leads 3% thereof passing upwardly through the hollow control rod drive shaft 52. The scram magnet coil 302 and core 9 304 is adapted to attract an armature 398 fabricated from magnetic material and secured to the upper end of a control rod supporting Itube fitti. Mounted adjacent the upper end of the supporting tube 310 is a conventional shock absorber 312 adapted to cooperate in association with the upper end of the c-ontrol rod shroud tube mi to `absorb at least part Iof the `shock of the descending control rod `assembly 6@ in the event the latter is scramrned. The shock absorber 312 includes, in this example, an upper cup member 313 rigidly secured to the control rod supporting tube Si@ and interlitting within a Vlower cup member 315 slidably mounted upon the supporting tube Slt). Shock energies resulting from the lower cup member 315 striking the upper end `of the upper shroud tube 9@ are absorbed by a spring 317 interposed Within the cup-shaped members k313 and 3?;5.

The control rod assembly is shown here in its scrammed or lowermost position, for purposes of illustration, whereat the neutron 'absorbing material contained within, and distributed along the length of, Eone or more tubular elements 3l@ of the control rod assembly is positioned substantially centrally of the end pla-tes 68 and 70 of the reactor core 30. Thus, the neutron absorbing tube 31d, at the aforesaidV scrammed position `of the control rod assembly, is supported adjacent the core section 23d of the fuel elements 6d, las illustrated by one of the fuel elements 64a shown in FIG. 7. The neutron absorbing tube Elliis supported, in a manner presently to be elaboratedrated upon, between -an upper, spoked bracket 3io and a modified fuel element bracket Ztiea having suitable indentations or shoulders 318 formed in the spokes 319 thereof to receive Iand engage the `lower end of the neutron absorbing tube 314.

For purposes of preventing or minimizing perturbations in the neutr-onic flux existing within the reactor core `3d, as control rod section 32d (FG. 8) comprising the neutron absorbing tube 314 (FIG. 7) is withdrawn from the reactor core iiti, a fuel element section 322, desirably similar in construction and nuclear characteristics to that of the fuel elements ofi, is secured in tandem to the lower end of the control rod section E2@ for movenient therewith. The fuel element sec-tion 322 therefore includes, in this example, a plurality of coaxially nested tubes indicated generally at 324 and secured in spaced relationship by bottom bracket and clamping member tibo and Elda, respectively, like the bracket 26S and clamping member `219.6 described heretofore in connection with FIGS. 4 6 of the drawings, and by the modified upper bracket tion and a lower connecting link 325 threadedly engaging a supporting tube section 326 positioned coaxially within the fuel element 322. The connecting link 325 is welded or otherwise attached at the other end thereof to a supporting tube section 32S disposed coaxially within the control rod section 32.9. The supporting tube section 328 is secured to the control rod assembly supporting tube section 3l@ by means of an upper connecting link 33t?. With the aforesaid disposition of the supporting tube section 323 within the neutron absorbing tube 3Std a channel 332 is formed therebetween for the passage of water or other coolant fluid. The channel 332 communicates through the spokes 3E? of the upper fuel element bracket Zitia, with passages 33d existing between the concentric tubes 324 and the supporting tube section 32.5 of the fuel element section 322.

ln the aforedescribed arrangement, when the control rod section 320 Iis withdrawn upwardly and out of the reactor core Si?, the fuel element section 322 is then positioned within the reactor core 30 adjacent the normally placed fuel elements 64. Therefore, the perturbations in the neutronic flux within the reactor core 30, caused by moving one or more of the control rod assemblies et) relative to the reactor core 3@ (VElGS. 2 and 3), are minimized or prevented altogether by an accompanying insertion or withdrawal of the individual fuel element sections `522 forming a part of the control rod assemblies 6b. When the contr-ol rod section 32d thereof is completely inserted into .the reactor core Sti, that is to say in the lowermost position of the control rod assembly ott, the associated fuel element section '322 then protrudes, Via enlarged lower end plate apertures Stia (BIG. 3), through the bottom plate 7d of the reactor core 30 and is suspended within the lower shroud tube 34, as better shown in FIG. 8.

Cooling of each control rod assembly ot?, whether the position of the assembly dit is such that either the neutron absorbing section 32@ or the fuel element section 322 is wholly or partially within the reactor core 30, is accomplished by the liow of coolant through the reactor vessel, as described heretofore relative to the reactor core sleeves do (FEGS. 1 3). More specifically, in the case of the control rod assemblies 60, the coolant enters the upward, open ends of the shroud tubes 9) (FIG. l), secured to the upper end plate 68 of the reactor core Sti; passes downwardly through each annular channel 336 between the control rod supporting tube section Siti and the shroud tube 9d; tlows between spokes 33S of the upper neutron absorbing tube bracket 3io into the communicating neutron absorbing section channel 332 and the fuel element section passages 334, described in the preceding paragraph; and thence passes between the spokes 2160. of the bottom fuel element bracket 293:1 and out of the reactor core l3o via the open lower end (FIG. l) of the lower control rod shroud tube 34. The coolant is then withdrawn from the reactor vessel 2t?, as described heretofore, by way of the annular passages 138 and the outlet duct 139.

FG. 9 shows in greater detail one of the high pressure testing Ithimbles, alluded to more generally heretofore in connection with FIGS. l to 3 of the drawings. One arrangement for pressurizing, and maintaining the desired temperature in the high temperatur-e testing thimbles, for example, the central testing tliirnble 1&2 (FGS. 3 and 9) includes the inner and outer cylinders del) and 492 extending longitudinally through the tubular casing 35, and protruding through the lower end thereof, which casing is secured, as noted heretofore, to the bottom closure 44 of the .reactor vessel 2d. The outer sleeve db2 of the :testing thimble M2 is secured to the flange d4 of the tubular casing 35' by means of a collar ido welded to the exterior of the outer sleeve ed?, and by means of a number of bolt and spacer arrangements indicated generally by the reference character 4%.

Immediately below the collar d66 an inlet conduit 41) is provided for the purposes of admitting pressurized water or other `Huid into an annular space 4t2 enclosed between the inner and outer sleeves tdt) and 4tl2. The annular space i712 is terminated at the upper end thereof by the cap 4,15 of the testing thimble IGZ, and at the lower end thereof by a spacing collar li/ welded adjacent the lower end of the inner sleeve ddd. Suitable sealing means 4to is provided on the outer periphery of the collar iid to afford a water-tight engagement of the collar 4M with Ithe outer sleeve 462i. At a position adjacently below the collar die, inner sleeve doti is provided with an opening dit', which communicates Vith an outlet conduit 42d .and with an annular space 422 between the inner and outer sleeves and itt-l2, and limited by the collar 4M and a bottom closure 426i sealed to the lower end of the inner sleeve eetl. The bottom closure 42e forms a snug fit with the outer sleeve db2 and is hermetically sealed to a casing 426 surrounding suitable instrument wiring or tubing indicated schematically by leads 42?. The casing 426 is hermetically sealed to and supported by the lower end of the outer sleeve im by means of a suitable coupling and packing arrangement indicated generally by the reference character 43th.

During operation of the nuclear reactor, high pressure water or other iiuid is admitted into the annular space 4t2 by way of the conduit dit?, .as indicated by iiow arrows 432. The pressurized fluid then flows upwardly between the inner and outer `sleeves 400 and 402 and near the top of the testing -t-himble 102, iiows between radial supporting plates 434, which plates are arranged to space the upper end of the inner sleeve 400 rela-tive to the outer sleeve 402. The pressurized ii-uid then flows downwardly among testing specimens 436, which may take the form of concentric tubes supported, by means of brackets 43S and 440, within the inner sleeve `$00 at a position intermediate the reactor core end plates 6? and '70 (FiG. 3). The pressurized uid Ithen ows downwardly through the inner sleeve 400 as indicated by tlow arrows 442 and exits through the outlet conduit 42) by way of the aperture 418 4and annular spa-ce 422.

With .the testing arrangement illustrated in FIG. 9, it is apparent that materials to be associated with nuclear reactors can be tested lat high temperatures and pressures within the testing thimble 1102, without subjecting the balance of the reactor core 30 to the same 'high pressures and temperatures, thereby permitting the testing specimens 436 to be `subj'ected to conditions normally existing within la nuclear -power reactor while permitting the test reactor to be operated at lower temperatures Iand pressures of the moderator and coolant fluid. Obviously, the temperatures existing within the testing thimble 102 can be adjusted by controlling the rate of flow of the pressurized lluid owing therethrough Ito rem-ove more or less of the heat developed in the testing thimble 102 by the nuclear chain reaction within the reactor core 30. A similiar pressurizing arrangement is provided for each of the high pressure testing thimbles 11S shown in FIGS. 1 and 3 of the drawings and inserted individually into the tubular casings 35.

Turning now to FlGS. to 12 of .the drawings there is shown therein a manipulating tool adapted for use in handling the fuel elements 64 and the end plate plugs 1.36 (FIG. 3). The tool 500 is provided with la tubular handle 502 having sufficient length to permit lowering the `tool 500, through the protective head of coolant overlying the reactor core 30, to the upper end plate 68 of the reactor core, from the top of the reactor vessel 20. The operable portion of the tool 500 includes `a cylindrical 'housing 504 and a body mem-ber 506 threadedly engaging iat the upper end thereof the tubular handle 502. The Ahousing S04 is secured to the body member 506 in any convenient manner, as by shrink-tting, and loss of the housing into the reactor is made virtually impossible through the provision of flan-ge portions 507 formed integrally with the body member 566 adjacent the lower end thereof. Spaced around the circumference of the body member 506, Ithe iiange portions 507, being three in number .in this example, 'are each provided with a shoulder 508 for engaging the lower end of the housing 504.

The body member S06 is provided with a centrally extending channel 509 Iand with a plunality of slots 510 extending longitudinally of said body member and adjoining the central channel 509. inserted into each of the slots 510 is an operating lever 512 secured to body member 506 by means of .a pivot pin 514 inserted through the body member 506 at a position adjacent the recesses 516 provided about the outer periphery of lthe body member 506. Near the lower end of the 'body member 506, the channel S09 opens into a widened portion 518 thereof adapted to receive the spheroidal knob 221 of the fuel element cla-mp 2id or the spheroidal knob `1.37 of the plug members .136 (FlG. 3). To facilitate insertion of the knobs 221 or 137 into the widened portion 518, the body member 506 .is formed with a chamfer 520 at the lower end thereof and adjacent the widened channel portion 518. j

Near the llower end of each of the operating levers 512 ia projection l522 Iis provided and is adapted, upon manipuliation of the levers 512, in a manner presently to be described, to engage the undersurface 523 of the spheroidal 12 knobs 221 or 137 and to secure the knob within the widened channel .portion 518, in order to permit lifting thereof by the tool 500.

One arrangement for lmanipulating the levers 512 to cause the projections 522 thereof to eng-age the knob 221 or 137 inclu-des an operating extension 524 formed on each of the levers 512 adjacent the upper end .thereof and adapted to engage a tapered pin 526 which is mounted `for movement within the longitudinal channel 509. The pin 526 is coupled to an elongated connecting rod S28 extending through the elongated handle 502 land operated iat the upper end of the handle 502 by `a suitable mechanism including a gui-de rod portion 529 slidably `fitted within the tubular handle 502 and adjoining a knurled knob 530 and the connecting rod 523. The upper limit of movement of .the pin 525 and connecting rod 528 is determined by la stop pin 531 on the guide rod .portion 529 operating within a slot 532 disposed in the elongated handle 502 adjacent the upper end thereof. The pin 526 is furnished adjacent the upper end thereof with a connecting link 533, which Iis .threadedly secured to the connecting rod 1528 and is adapted to engage shoulder 534 of the body member 506 to limit the downward travel of the operating rod 52S :and pin 526. Upon withdrawal of the pin 526 'a cam surface 536 provid-ed on each of the projections 522 for moving in engagement with the undersurface 523 of the spheroidal knob 221 or i137 causes the lower end of the levers 512 to move outwardly and away from the channel 509, thereby effecting release of the spheroidal knob `221 `or 137 secured within the widened channel portion 518.`

In the following illustrative operating arrangement of the nuclear reactor forming the subject of the present invention it should be appreciated that the dimensions, masses, proportions and the like noted therein is presented by way of illustration only and not as substantive limitations upon the present invention. Assuming the -reactor is to have a peak thermal output of 20 megawatts (mw.), a cylindrical reactor core structure approximately 50 inches in height and 44 inches in diameter is utilized. As is noted heretofore, the upper and `lower end plates of the core are provided with like numbers of apertures arrange-d in a triangular lattice and spaced approximately 3 inches apart. Clustered about the central portion of the reactor core 30 are 51 fuel elements 64, each comprising three concentric tubes 200, 202 and 204, as described heretofore and each fuel element including a total of 200 ygrams of enriched uranium. For testing 'applications of the reactor a pressure of 125 pounds per square inch can be maintained within the reactor core 30 in order to lprevent boiling of the coolant and moderator iluids, which in this case are both light water. In accordance with the aforedescribed conditions yapproximately 14,000 gallons per minute of cooling water is required, which yields a temperature differential within the reactor core in the neighborhood of 10 F. As pointed out in the aforegoing description a total of nine control rod assemblies 60 are utilized to control the nuclear chain reaction within the reactor core 30, which assemblies are provided altogether with a quantity of neutron absorbing material, for example, eleven pounds of cadmium per rod, more than sufficient to terminate the aforesaid chain reaction when the control rods are completely inserted into the reactor core.

When operating the disclosed atomic reactor for power purposes in case of light water being used as a coolant and heavy water for a moderator, pressures of 2,000 to 2,500 pounds per square inch and temperatures of 500 to 600 F. can be maintained within the reactor core. The quantity of coolant utilized can be substantially equivalent to the rate of ow noted in the preceding paragraph, which, however, in the latter case will yield a temperature diiferential of approximately 25 F. within the reactor core, where an equivalent amount of atomic fuel is used.

Because the subject matter of the present invention involves a frontier, or a comparatively new art, a more detailed description of the structure thereof is given than is actually necessary to enable one skilled in this art to follow the teachings of the invention. It is to be understood therefore that the foregoing structural details are presented, not by way of limitation of the invention, but merely as suggestive examples of a few of many applications of the invention. Therefore, it is contemplated that numerous modications of the invention will occur to those versed in the art without departing from the scope of the appended claims. It will be understood further, and it is obvious from the foregoing detailed description of the invention, that certain features thereof can be utilized without a corresponding use of other features.

I claim as my invention:

1. A nuclear reactor comprising a hermetically sealed vessel; a reactor core casing supported within said vessel, said casing having oppositely disposed and aligned lattices of apertures therein; a plurality of reactor core sleeves secured within said casing and communicating with aligned pairs of said apertures; a number of atomic fuel elements positioned individually within some of `said reactor core sleeves; said fuel elements each including at least one open-ended tubular member supported within the associated reactor core sleeve and spaced therefrom and a mass of atomic fuel forming part of said tubular member; a number of control rod assemblies individually passing through at least some of the remaining sleeves; said assemblies each including at least one tubular section disposed generally within said core casing; means for moving said control rod assemblies relative to saidreactor core casing; and means for circulating cooling uid through said reactor core sleeves, said fuel element tubular members, and said control rod tubular sections.

2, A nuclear reactor comprising la hermetically sealed vessel; a reactor core structure supported within said vessel, said reactor core having a casing including a pair of end plates, said end plates each having a lattice of apertures therein aligned with the apertures in the other of said end plates; a plurality of reactor core sleeves disposed within said casing and -secured to said end plates and communicating with aligned pairs of said apertures; a number of atomic fuel elements inserted individually into some of said reactor core sleeves, each said elements being positioned within said reactor core by engagement with at least one of said end plates; a number of control rod assemblies individually inserted through at least some of the remaining sleeves; means for moving said control rod assemblies relative to said reactor core; and means for circulating cooling fluid through said reactor core sleeves.

3. A nuclear reactor comprising a hermetically sealed vessel; a reactor core casing supported within said vessel, said casing having oppositely disposed and aligned lattices of apertures therein; a plurality of reactor core sleeves secured within said casing and communicating with aligned pairs of said apertures; a number of nuclear fuel elements positioned individually in some of said reactor core sleeves; a number of control rod assemblies individually inserted through at least some of the remaining sleeves, said control -rod assemblies each including a neutron absorbing section and a nuclear fuel section connected in tandem thereto, said fuel section having substantially the same nuclear and structural formation as that of said reactor fuel elements; means for moving said control rod assemblies relative to said reactor core casing; and means for circulating cooling uid through said reactor core sleeves.

4. A nuclear reactor comprising a hermetically sealed vessel; a reactor core casing supported within said vessel; a plurality of reactor core sleeves passing through said casing and `secured thereto, said sleeves communicating with the exterior of said casing; a number of atomic fuel elements positioned individually in some of `said reactor core sleeves; a number of control rod assemblies ndividually insertable through others of said reactor core sleeves; means for moving said control rod assemblies relative to said reactor core; means for circulating cooling liuid through said reactor core sleeves; at least one testing thimble casing passing through said reactor vessel wall and hermetically sealed thereto, said testing thimble casing extending into said reactor core casing through at least one of the remainder of said sleeves; and a hermetically sealed testing thimble insertable through said testing thimble casing to a position within said reactor core.

S. A nuclear testing reactor comprising a sealed vessel, a plurality of fuel elements, means for mounting said elements within said vessel in a spaced array, each of said fuel elements including at least one open-ended tubular member and a mass of atomic fuel forming part of `said tubular member, a number of control rod assemblies, means for inserting and withdrawing said control rod assemblies relative to said array of fuel elements, at least one testing thimble casing passing through a wall of Said vessel and sealed thereto at such passage, said casing eX- tending into said array of fuel elements, a hermetically sealed testing thirnble mounted within said casing and movable to a position within said casing adjacent said fuel element array, means coupled to said Vessel for circulating cooling uid through said fuel element arra and means coupled to said testing thirnble for circulating coolant therethrough.

6. A nuclear reactor comprising a sealed vessel; a reactor core casing supported within said vessel, said casing having oppositely disposed and aligned pairs of apertures therein, a plurality of reactor core sleeves secured within said casing and sealably communicating with said pairs of apertures respectively, a number of nuclear fuel elements positioned individually within some of said reactor core sleeves, said fuel elements each including at least one openended tubular member supported within the associated reactor core sleeve and spaced therefrom and a mass of nuclear fuel forming part of said tubular member, a number of Control rod assemblies, means for moving said control rod assemblies through at least some ofthe remaining reactor core sleeves respectively, said control rod assen blies each comprising tubular neutron-absorbing section and at least one open-ended tubular segment joined tandemly to said section in communication therewith, said segment having a mass of nuclear fuel forming a part thereof and means for circulating cooling uid through said reactor core sieeves, said fuel element tubular members, said control rod neutron absorbing tubular sections, and said control rod fuel bearing tubular segments.

7. A nuclear reactor comprising a sealed vessel; a plurality of nuclear fuel elements; the two end portions of each fuel element being identical; means for supporting said fuel elements within said vessel in a spaced array; said fuel elements each including a plurality of nested tubes, a mass of nuclear fuel material forming part of each of said tubes, and means for clamping said tubes in a laterally and coaxially spaced relationship with one another; a plurality of control rod assemblies; means for inserting and withdrawing said assemblies relative to said fuel element arra and means for circulating cooling fluid through said fuel element array and through said nested tubes.

8. A nuclear reactor comprising a hermetically sealed vessel; a reactor core casing supported within said vessel, said casing having oppositely disposed and aligned lattices of apertures therein; a pluraiity of reactor core sleeves disposed within said casing and sealably secured adjacent their ends to said casing, said sleeves communicating with aligned parts of said apertures respectively; a number of fuel elements containing fissile material and positioned individually within some of said reactor core sleeves; a number of control rod assemblies individually passing through at least some of the remaining sleeves, the sleeves thus associated with said assemblies being dispersed among the iii sleeves containing said fuel elements; means for moving said control rod assemblies relative to said associated sleeves and to said fuel elements; means for circulating cooling fluid through said reactor core sleeves to remove heat developed in said fuel elements and said assemblies; and means coupled to said core casing for circulating moderating fluid through said core casing but exteriorly of said core sleeves.

9. In a nuclear reactor core having a number of fissile fuel elements supported within an enclosure and cooling and moderating material surrounding said fuel elements, the combination comprising a reactor core enclosure having a number of channels extending therethrough, and a plurality of control rod assemblies insertable through said channels, said assemblies each including a neutron absorbing section and a nuclear fuel section connected in tandem thereto, said fuel section having substantially the same nuclear and structural formation as that of said fuel elements, said fuel sections and said fuel elements each having coolant passages extending generally longitudinally therethrough, the coolant passages of said fuel sections being of substantially the same Configuration and size as those of said fuel elements, and means for moving said control rod assemblies relative to said fuel elements.

it). A nuclear reactor comprising a sealed vessel, a plurality of fuel elements, means for mounting said elements within said vessel in a spaced array, each of said fuel elements including at least one open-ended tubular member and a mass of nuclear fuel forming part of said tubular member, a number of control rod assemblies, means for inserting and withdrawing said control rod assemblies relative to said array of fuel elements, each of said control rod assemblies including a tubular neutronabsorbing section and at least one open-ended tubular member joined to and communicating with said section and having a mass of nuclear fuel forming part thereof, coolant directing means mounted in said core and individually and sealably surrounding each of said fuel elements and said control rod assemblies, and means for circulating cooling fluid through said coolant directing means,

'said fuel elements and said control rod assemblies.

ll. A nuclear reactor comprising a hermetically sealed vessel, a reactor core casing supported within said vessel, said casing having oppositely disposed and aligned lattices of apertures therein, a plurality of reactor core sleeves mounted within said casing and sealably communicating with aligned pairs of said apertures, a number of nuclear fuel elements positioned individually within some of said reactor core sleeves, said fuel element each including a plurality of nested tubes, a mass of nuclear fuel material forming part of each of said tubes, and means for clamping said tubes in laterally spaced relationship with one another, a plurality of control rod assemblies mounted for individual insertion through at least some of the remaining sleeves, means for inserting and withdrawing said assemblies relative to their associated sleeves and to said fuel elements, means for circulating cooling uid through said sleeves and through said fuel element nested tubes, and means for circulating moderator fluid around and outside of said sleeves.

l2. A nuclear reactor comprising a hermetically sealed vessel; a reactor core casing supported Within said vessel, said casing having oppositely disposed and aligned lattices of apertures therein; a plurality of reactor core sleeves secured within said casing and sealably communicating with aligned pairs of said apertures, respectively; a number of nuclear fuel elements positioned individually within some of said reactor core sleeves, said fuel elements each including a plurality of nested tubes, a mass of nuclear fuel material forming part of each of said tubes, and means for clamping said tubes in laterally spaced relationship with one another; a number of control rod assemblies individually insel-table through at least some of the remaining sleeves, said control rod assemblies each including a tubular neutron absorbing section, a plurality of lati nested tubular segments joined tandemly to said tubular section in communication therewith, a mass of nuclear fuel material forming part of each of said tubular segments, and means for clamping said segments in laterally spaced relationship; at least one sealed testing thimble casing passing through a wall of said vessel and sealed thereto at such passage, said casing extending into said core to a position adjacent said fuel elements; a hermetically sealed testing thimble movably mounted within said casing for movement therewith to a position adjacent said fuel elements; means coupled to said vessel for circulating cooling uid through said sleeves, said fuel elements, and said control rod assemblies; and means coupled to said testing thimble for circulating cooling fluid therethrough.

i3. A nuclear reactor comprising a hermetically sealed vessel, a reactor core casing supported within said vessel, said casing having oppositely disposed and aligned lattices of apertures therein, a plurality of reactor core sleeves mounted within said Casing and sealably communicating with aligned pairs respectively of said apertures, a number of nuclear fuel elements positioned individually within some of said reactor core sleeves, a number of control rod assemblies individually insertable through at least some of the remaining sleeves, said assemblies each including a neutron absorbing section and a nuclear fuel section connected in tandem thereto, said fuel section having substantially the same nuclear and structural formation as that of said fuel elements, said fuel sections and said fuel elements each having coolant passages extending generally longitudinally therethrough, the coolant passages of said fuel sections being of substantially the same configuration and size as those of said fuel elements, means for moving said control rod assemblies relative to said fuel elements, means for circulating coolant fluid through said core sleeves and through said passages, and means for circulating moderating fluid through said core casing but exteriorly of said core sleeves.

14. A nuclear testing reactor comprising a hermetically sealed vessel; a reactor core structure supported within said vessel, said reactor core having a casing including an upper end plate and a lower end plate, said end plates each having a lattice of apertures therein aligned with the apertures in the other of said end plates; a plurality of reactor core sleeves sealably secured within said casing and communicating with aligned pairs of said apertures, respectively; a number of nuclear fuel elements positioned individually within some of said reactor core sleeves and supported by said bottom end plate, said fuel elements each including a plurality of substantially coaxially nested tubes and having identical end portions; a number of control rod assemblies individually insertable through at least some of the remaining reactor core sleeves, said control rod assemblies each including a tubular neutron absorbing section and a nuclear fuel section connected in tandem thereto, said fuel section having substantially the same nuclear and structural formation as that of said fuel elements, said fuel sections and said fuel elements each having coolant passages extending generally longitudinally therethrough, the coolant passages of said fuel sections being of substantially the same configuration and size as those of said fuel elements, means for moving said control rod assemblies relative to said fuel elements; at least one sealed testing thimble casing passing through a wall of said vessel and sealed thereto at such passage, said testing thimble casing extending through said core to a position adjacent said fuel elements; a hermetically sealed testing thimble movably mounted within said testing thimble casing for movement therewithin to a position adjacent said fuel elements, said testing thimble including substantially coextensive inner and outer coaxial cylinders, and means for circulating fluid through aid testing thimble;

means coupled to said vessel for circulating cooling fluidl through said reactor core sleeves, said fuel elements and f' said control rod assemblies; and means for circulating moderating uid through said core casing but exteriorly of said core sleeves.

References Cited by the Examiner UNITED STATES PATENTS Flynn 214-23 Fischer et al. Z50-50 Crosby 214-23 Reyengs 250-50 Gifford et al Z50-108 X Fermi et al. 176-41 Metcalf et al. 176-45 10 CARL D. QUARFORTH, Primary Examiner.

LEON D. ROSDOL, Examiner. 

1. A NUCLEAR REACTOR COMPRISING A HERMETICALLY SEALED VESSEL; A REACTOR CORE CASING SUPPORTED WITHIN SAID VESSEL, SAID CASING HAVING OPPOSITELY DISPOSED AND ALIGNED LATTICES OF APERTURES THEREIN; A PLURALITY OF REACTOR CORE SLEEVES SECURED WITHIN SAID CASING AND COMMUNICATING WITH ALIGNED PAIRS OF SAID APERTURES; A NUMBER OF ATOMIC FUEL ELEMENTS POSITIONED INDIVIDUALLY WITHIN SOME OF SAID REACTOR CORE SLEEVES; SAID FUEL ELEMENTS EACH INCLUDING AT LEAST ONE OPEN-ENDED TUBULAR MEMBER SUPPORTED WITHIN THE ASSOCIATED REACTOR CORE SLEEVE AND SPACED THEREFROM AND A MASS OF ATOMIC FUEL FORMING PART OF SAID TUBULAR MEMBER; A NUMBER OF CONTROL ROD ASSEMBLIES INDIVIDUALLY PASSING THROUGH AT LEAST SOME OF THE REMAINING SLEEVES; SAID ASSEMBLIES EACH INCLUDING AT LEAST ONE TUBULAR SECTION DISPOSED GENERALLY WITHIN SAID CORE CASING; MEANS FOR MOVING SAID CONTROL ROD ASSEMBLIES RELATIVE TO SAID REACTOR CORE CASING; AND MEANS FOR CIRCULATING COOLING FLUID THROUGH SAID REACTOR CORE SLEEVES, SAID FUEL ELEMENT TUBULAR MEMBERS, AND SAID CONTROL ROD TUBULAR SECTIONS. 